1. Field of the Invention
The present invention relates to vehicle heating systems, and more particularly, to those employing a viscous fluid heat generator.
2. Description of the Related Art
Viscous fluid type heat generators agitate viscous fluid to generate heat. Viscous fluid heat generators are compact and efficient. Thus, viscous fluid type heat generators are often used in vehicles, which have limited space.
Japanese Examined Utility Model Publication No. 7-52722 describes a vehicle heating system including a viscous fluid type heat generator, a coolant temperature sensor and a control circuit. An engine drives the heat generator by means of an electromagnetic clutch. The temperature sensor is provided in a passage through which coolant flows. The coolant cools the engine, or absorbs the heat of the engine. The control circuit controls the electromagnetic clutch in accordance with the temperature detected by the temperature sensor. If the coolant temperature is equal to or lower than a predetermined first threshold temperature, the control circuit activates the electromagnetic clutch and causes the heat generator to generate heat. If the coolant temperature is equal to or higher than a predetermined second threshold temperature, the control circuit deactivates the clutch and stops the generation of heat from the heat generator. Accordingly, the viscous fluid type heat generator serves as an auxiliary heater and heats the passenger compartment when the coolant temperature is low.
There to another type of viscous fluid type heat generator that employs a single predetermined threshold temperature. This heat generator selectively activates and deactivates an electromagnetic clutch based on a single predetermined threshold temperature. Changes in the coolant temperature are detected and compared with the threshold temperature to determine whether to activate or deactivate the electromagnetic clutch. If the detected temperature falls below the threshold temperature, the clutch is activated and the heat generator generates heat. Accordingly, the coolant temperature starts to increase. Contrarily, if the detected temperature surpasses the threshold temperature, the clutch is deactivated and the heat generator stops generating heat. Accordingly, the coolant temperature starts to fall. This results in frequent activation and deactivation of the electromagnetic clutch, and thereby causes frequent and abrupt fluctuations of the engine's load torque. This makes the driver uncomfortable.
Using two different predetermined threshold temperatures (T, T-.DELTA.T) reduces frequent activation and deactivation of the electromagnetic clutch. .DELTA.T is the difference between the two threshold temperatures. However, the detected temperature is compared with the temperatures T -.DELTA.T, T in an electric control circuit. This complicates the structure of the control circuit and requires an electrical circuit for activating the electromagnetic clutch. Accordingly, the system is troublesome and costly.